There are many closures available on the market, but each of them is always to be used only in a specific case. There are closures for tubes, closures for bottles, closures for cans, etc.
Besides this classification based upon their application, the closures can also be divided according to the way they function, such as rotatable closures, snap closures and press closures. The closures currently used are mostly formed as sheet metal caps or are formed from synthetic-resin material.
An ideal synthetic-resin closure would have to fulfill the following partly contradictory requirements:
1. THE CLOSURE MUST SEAL TIGHTLY, BUT BE EASILY OPENABLE, WHEN POSSIBLE, BY ONE HAND;
2. THE CLOSURE SHOULD BE CAPABLE OF WITHSTANDING A CERTAIN INTERNAL PRESSURE AS ARISES ESPECIALLY IN THE CASE OF CARBONATED BEVERAGES;
3. THE CLOSURE SHOULD BE CAPABLE OF WITHSTANDING TEMPERATURES OF THE TYPE ARISING DURING PASTEURIZATION (62.degree. TO 85.degree. C.);
4. the closure should be able to be made as a child-proof unit, without major modification;
5. THE CLOSURE SHOULD BE CAPABLE OF SEALING TIGHTLY GLASS BOTTLES HAVING A WIDE RANGE OF TOLERANCES AT THEIR MOUTHS; AND
6. THE CLOSURE SHOULD, WITHOUT MAJOR MODIFICATIONS, BE ABLE TO BE PROVIDED WITH A SEALING BAND.
All of the conventional closures have the disadvantage that they fulfill at most four of the above-listed requirements. Particularly, to date there has not been developed a closure in which a cover and a base are connected to each other and which is capable of simultaneously sealing the container against internal pressure and withstanding high temperatures. In the case of pressure-tight closures, it is usually difficult to open the closure with one hand without applying a high degree of force to the container. The previously known closures can be applied to solving different closure problems only after major modifications. Pressure-tight closures, for instance, can hardly be used for closing pressureless containers which must be opened and closed repeatedly.